How to create texture for sculpting from mesh
You may render the AO of your sculpt and use it as an alpha texture for sculpting. Though I don't think it's a perfect method it gives a decent result.

I've quickly sculped some rocks to show you an example.

First remove all the lamps from your scene. Then add a material to your sculpt and set its specularity to 0.000

Select your camera and set its type to Ortographic. Set the Ortographic Scale to fit the sculpt borders as you want.

Go to World header, set the Horizon Color to black. Check the Ambient Occlusion checkbox (Add, Factor 1.000). In Gather panel choose the Raytrace and set the Samples to desired amount (the more samples, the more detailed alpha texture for sculpting'll be). Render, then save rendered image.

Now load your rendered image as an alpha texture in the Texture panel of the Sculpt Mode.

I used the Stencil mapping type with a harsh curve type to sculpt, but you may experiment with it.

NOTE: You may try to bake the AO (from the sculpt to a flat plane [selected to active]), save the result and then use it as the alpha mask for sculpting. I haven't tried this method though.